Try to understand the error (it is actually quite informative).
It tells you that in this line:
you’re trying to add something that is of type Type{KernelFunctions.Kernel} to something of type LinearAlgebra.UniformScaling{Float64}.
Is this what you expect? No. You want the first summand to be of type Matrix{Float64}.
In other words: Your K is not a matrix (you said you deleted the line, but you didn’t say what you replaced it with).
My general advice is: Try to understand the errors and go step by step from there (e.g. make sure you understand every line and the output of every line before continuing iterating on the full script/model)